Items such as that disclosed in, for example, patent document 1 have been proposed as variable volume type water pumps conventionally used in engines mounted in vehicles and the like. Patent document 1 discloses a water pump wherein a first rotation member (drive-end rotation member) whereto a water pump pulley is fixed and a second rotation member (driven-end rotation member) whereto a pump impeller is fixed are connected via a multiplate wet clutch having a viscous fluid as a medium. Furthermore, provision inside a cooling water channel of a temperature sensitive member deforming according to a temperature of cooling water in order to disconnect the multiplate wet clutch is disclosed. The water pump specified in this patent document 1 is configured such that, when a water temperature is low, driving of the water pump is substantially stopped in order to reduce friction and prevent deterioration of fuel efficiency, and furthermore, when a water temperature is high, the clutch is set to an engaged condition and rotation of the first rotation member is transmitted to the second rotation member.
In addition, items wherein transmission of rotation from the drive-end rotation member to the driven-end rotation member is carried out in a non-contact condition have also been proposed as variable volume type water pumps. The components of this water pump related to the transmission of rotation from the drive-end rotation member to the driven-end rotation member are shown in FIG. 4.
As shown in FIG. 4, an interval between a drive-end rotation member 101 and a driven-end rotation member 103 is partitioned by a dividing wall 105. In addition, a permanent magnet 102 mounted on the drive-end rotation member 101 and an induction ring 104 mounted on the driven-end rotation member 103 are provided so as to be opposed with a prescribed interval therebetween. The induction ring 104 is configured having an aluminum ring member 104b mounted on an outer periphery of a magnetic core 104a. When the drive-end rotation member 101 rotates, the magnetic field of the permanent magnet 102 acting on the induction ring 104 changes. As a result of this, an induction current in a direction obstructing that magnetic field change is generated in the ring member 104b of the induction ring 104. A torque is generated in the ring member 104b of the induction ring 104 pursuant to this induction-current generation. As a result, the driven-end rotation member 103 rotates and the water pump drives.
Furthermore, the torque transmitted to the driven-end rotation member 103 is changed by changing an overlap amount (degree of mutual overlap in the axial direction) L2 of the permanent magnet 102 of the drive-end rotation member 101 and the ring member 104b of the induction ring 104 in an axial direction (rotation axis direction). As a result, modification of a pump flow volume of the water pump is possible.    Patent document 1: JP2001-90537